Peel and slice onions to about 1/8-inch thick — thin enough to cook quickly, thick enough to avoid scorching. Yellow or Spanish onions work best. Limit to 2 large onions per 12-inch Skillet to prevent steaming. Have your fat ready and keep a splash of water, broth, or wine nearby to deglaze if needed.

Heat a Skillet over medium to medium-high heat. Add butter, oil, or a mix and let it shimmer. Add sliced onions in large handfuls, stirring for 1–2 minutes per batch until they begin to soften — this prevents overcrowding. Once all onions are in, season with a pinch of salt. Cook over medium heat, stirring every few minutes, for 8–12 minutes total until soft, translucent, and lightly golden. To speed up softening, add 1/4 to ½ cup water or broth and cover; steam wilts the onions in a few minutes. Remove the lid once liquid evaporates and continue cooking uncovered. If the pan looks dry, deglaze with a splash of water, broth, or wine, scraping up any browned bits. Don't crank the heat — high heat burns the outside before the onions cook through.

Drizzle 1 tablespoon of 5 tbsp extra-virgin olive oil divided into a clean large Mixing Bowl. Transfer the dough into the bowl and turn it to coat in the oil. Cover tightly with Plastic Wrap or a damp Kitchen Towel. Let rise at room temperature for 1 to 1.5 hours, until doubled in size.

Pour the remaining 1 tablespoon of 5 tbsp extra-virgin olive oil divided into a Baking Dish (9x13-inch) and spread it around to coat the bottom and sides generously. Transfer the risen dough into the pan. Use your fingertips to gently stretch and press the dough toward the edges of the pan. If it springs back, let it rest for 5 minutes and try again — don't force it.

Cover the pan loosely with Plastic Wrap and let the dough rest for another 20–30 minutes, until it puffs up slightly and fills the pan. While it rests, preheat your Oven to 450°F with a rack in the center.

Drizzle the 3 tablespoons of 5 tbsp extra-virgin olive oil divided evenly over the top of the dough. Oil your fingers well, then press straight down into the dough firmly and confidently to create deep dimples all across the surface — go all the way to the bottom of the pan. These dimples are the signature of focaccia and they hold the oil and toppings beautifully.

Scatter the sliced 3 cloves garlic thinly sliced and chopped 2 tsp fresh rosemary roughly chopped (if using) evenly across the surface, pressing them lightly into the dimples. Finish with a generous, even sprinkle of 2 tsp kosher salt over the entire surface.

Bake at 450°F for 22–26 minutes, until the top is deep golden brown and the edges are pulling away from the sides of the pan. The bottom should be golden and crispy — you can lift a corner with a Spatula to check.

Remove from the Oven and let cool in the pan for 5 minutes, then transfer to a Wire Rack. Slice into squares and serve warm. For the crispiest bottom, don't let it sit in the pan as it cools.